This invention generally relates to coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a ceramic coating for such components that exhibits low thermal conductivity and resistance to spallation.
Components within the hot gas path of gas turbine engines are often protected by a ceramic coating, commonly referred to as a thermal barrier coating (TBC). TBC's are typically formed of ceramic materials deposited by thermal spraying and physical vapor deposition (PVD) techniques. Thermal spraying techniques, which include plasma spraying (air, vacuum and low pressure) and high velocity oxy-fuel (HVOF), deposit TBC material in the form of molten “splats,” resulting in a TBC characterized by noncolumnar, irregular flattened grains and a degree of inhomogeneity and porosity. TBC's employed in the highest temperature regions of gas turbine engines are most often deposited by PVD, particularly electron-beam PVD (EBPVD), which yields a porous, strain-tolerant columnar grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., laser melting, etc.).
Various ceramic materials have been proposed as TBC's, the most widely used being zirconia (ZrO2) partially or fully stabilized by yttria (Y2O3), magnesia (MgO), or ceria (CeO2) to yield a tetragonal crystal structure that resists phase changes. Other stabilizers have been proposed for zirconia, including hafnia (HfO2) (U.S. Pat. No. 5,643,474 to Sangeeta), gadolinium oxide (gadolinia; Gd2O3) (U.S. Pat. Nos. 6,177,200 and 6,284,323 to Maloney), and dysprosia (Dy2O3), erbia (Er2O3), neodymia (Nd2O3), samarium oxide (Sm2O3), and ytterbia (Yb2O3) (U.S. Pat. No. 6,890,668 to Bruce et al.). Still other proposed TBC materials include ceramic materials with the pyrochlore structure A2B2O7, where A is lanthanum, gadolinium or yttrium and B is zirconium, hafnium and has been the most widely used TBC material. Reasons for this preference for YSZ are believed to include its high temperature capability, low thermal conductivity, and relative ease of deposition by thermal spraying and PVD techniques.
TBC materials that have lower thermal conductivities than YSZ offer a variety of advantages, including the ability to operate a gas turbine engine at higher temperatures, increased part durability, reduced parasitic cooling losses, and reduced part weight if a thinner TBC can be used. As is known in the art, conventional practice is to stabilize zirconia with yttria (or another of the above-noted oxides) to inhibit a tetragonal to monoclinic phase transformation at about 1000° C., which results in a volume expansion that can cause spallation. At room temperature, the more stable tetragonal phase is obtained and the undesirable monoclinic phase is minimized if zirconia is stabilized by at least about six weight percent yttria. An yttria content of seventeen weight percent or more ensures a fully stable cubic (fluorite-type) phase. Though the thermal conductivity of YSZ decreases with increasing yttria content, the conventional practice has been to partially stabilize zirconia with six to eight weight percent yttria (6-8% YSZ) to promote spallation resistance. As such, ternary systems have been proposed to reduce the thermal conductivity of YSZ. For example, commonly-assigned U.S. Pat. No. 6,586,115 to Rigney et al. discloses a YSZ TBC alloyed to contain an additional oxide that lowers the thermal conductivity of the base YSZ composition by increasing crystallographic defects and/or lattice strains. These additional oxides include alkaline-earth metal oxides (magnesia, calcia (CaO), strontia (SrO) and barium oxide (BaO)), rare-earth metal oxides (ceria, gadolinia, neodymia, dysprosia and lanthana (La2O3)), and/or such metal oxides as nickel oxide (NiO), ferric oxide (Fe2O3), cobaltous oxide (CoO), and scandium oxide (Sc2O3). Another ternary YSZ coating system that exhibits both reduced and more stable thermal conductivity is YSZ+ niobia (Nb2O3) or titania (TiO2), as disclosed in U.S. Pat. No. 6,686,060 to Bruce et al. Finally, U.S. Pat. No. 6,025,078 to Rickerby et al. discloses YSZ modified to contain at least five weight percent gadolinia, dysprosia, erbia, europia (Eu2O3), praseodymia (Pr2O3), urania (UO2), or ytterbia to reduce phonon thermal conductivity.
Additions of oxides to YSZ coating systems have also been proposed for purposes other than lower thermal conductivity. For example, U.S. Pat. No. 6,352,788 to Bruce teaches that YSZ containing about one up to less than six weight percent yttria in combination with magnesia and/or hafnia exhibits improved impact resistance. In addition, U.S. Pat. No. 7,060,365 to Bruce discloses that small additions of lanthana, neodymia and/or tantala to zirconia partially stabilized by about four weight percent yttria (4% YSZ) can improve the impact and erosion resistance of 4% YSZ. U.S. Pat. No. 4,753,902 to Ketcham discloses sintered zirconia-based ceramic materials containing yttria or a rare-earth metal oxide as a stabilizer and further containing at least five molar percent (about 3.0 weight percent) titania for the purpose of minimizing the amount of stabilizer required to maintain the tetragonal phase. Finally, U.S. Pat. No. 4,774,150 to Amano et al. discloses that bismuth oxide (Bi2O3), titania, terbia (Tb4O7), europia and/or samarium oxide may be added to certain layers of a YSZ TBC for the purpose of serving as “luminous activators.”
The service life of a TBC system is typically limited by a spallation event brought on by thermal fatigue, which results from thermal cycling and the different coefficients of thermal expansion (CTE) between ceramic materials and the metallic bond coat and substrate materials on which they are deposited. An oxidation-resistant bond coat is often employed to promote adhesion and extend the service life of a TBC, as well as protect the underlying substrate from damage by oxidation and hot corrosion attack. Bond coats used on superalloy substrates are typically in the form of an overlay coating such as MCrAIX (where M is iron, cobalt and/or nickel, and X is yttrium or a rare-earth element), or a diffusion aluminide coating. During the deposition of the ceramic TBC and subsequent exposures to high temperatures, such as during engine operation, these bond coats form a tightly adherent alumina (Al2O3) layer or scale that adheres the TBC to the bond coat.
Though considerable advances in TBC materials have been achieved as noted above, there remains a need for improved TBC materials that exhibit both low thermal conductivities and resistance to spallation.